Remove service registration APIs from public ICU API
At the suggestion of the ICU team to enable them to potentially drop the
behavior in a future ICU release.
Bug: 28583844
(cherry picked from commit ee0f20f8e03a1df95ced34550ed1b0ee06238eca)
Change-Id: Idcd101658976e7f23093b09752b9b2dae95589f7